G. J. Brown

Air Force Research Laboratory, Washington, Washington, D.C., United States

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Publications (123)206.37 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: This work report new integrated high quality factor (Q) GHz magnetic transformers based on solenoid structures with FeGaB/Al2O3 multilayer films. These transformers show excellent high-frequency performance with a wide operation frequency range of 0.5-5 GHz, in which primary, secondary, and mutual inductances are flat, and the peak quality factor can reach around 14 at frequency of 1.2 GHz. High mutual coupling and low insertion loss are also demonstrated. These novel GHz transformers with high Q and mutual coupling show great promise for applications in radio frequency integrated circuits.
    Journal of Applied Physics 04/2014; 115(17). · 2.21 Impact Factor
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    ABSTRACT: We report low-temperature spin spray deposited Fe3O4/ZnO thin film microwave magnetic/piezoelectric magnetoelectric heterostructures. A voltage induced effective ferromagnetic resonance field of 14 Oe was realized in Fe3O4/ZnO magnetoelectric (ME) heterostructures. Compared with most thin film magnetoelectric heterostructures prepared by high temperature (>600 °C) deposition methods, for example, pulsed laser deposition, molecular beam epitaxy, or sputtering, Fe3O4/ZnO ME heterostructures have much lower deposition temperature (<100 °C) at a much lower cost and less energy dissipation, which can be readily integrated in different integrated circuits.
    Journal of Materials Science Materials in Electronics 03/2014; 25(3). · 1.49 Impact Factor
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    ABSTRACT: We report new high quality factor (Q) integrated GHz magnetic inductors based on solenoid structures with FeGaB/ ${rm Al}_{2}{rm O}_{3}$ multilayer films, which show significantly enhanced inductance and quality factor at GHz frequencies over their air core counterparts. These inductors show an excellent high-frequency performance with a wide operation frequency range 0.5–2.5 GHz, in which the inductance is flat and the peak quality factor can reach ${sim}{rm 20}$ . The inductance of the magnetic inductor shows ${>}{100%}$ enhancement compared with that of the same size air core inductor. These novel GHz inductors with high inductance and Q enhancement show great promise for applications in radio frequency integrated circuits.
    IEEE Transactions on Electron Devices 01/2014; 61(5):1470-1476. · 2.06 Impact Factor
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    ABSTRACT: The InAs/InAsSb type-II superlattice materials studied to date for infrared detector applications have been residually n-type, but p-type absorber regions with minority carrier electrons can result in increased photodiode quantum efficiency, RoA, and detectivity. Therefore, Be-doped InAs/InAsSb superlattices were investigated to determine the p-type InAs/InAsSb superlattice material transport properties essential to developing high quality photodiode absorber materials. Hall measurements performed at 10 K revealed that the superlattice converted to p-type with Be-doping of 3 × 1016 cm-3 and the hole mobility reached 24 400 cm2/Vs. Photoresponse measurements at 10 K confirmed the 175 meV bandgap and material optical quality.
    Applied Physics Letters 12/2013; 104(1). · 3.52 Impact Factor
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    ABSTRACT: In order to limit cooling requirements, it is important to operate superlattice devices such as infrared detectors at the highest possible temperatures consistent with maintaining satisfactory figures of merit regarding signal and noise. One of the characteristics governing the device performance is vertical carrier mobility, although only horizontal mobilities are routinely measured. Recently, we calculated low-temperature vertical and horizontal mobilities, as limited by interface roughness scattering, for type-II InAs/GaSb superlattices as a function of SL dimensions and the degree of roughness. We found that the horizontal mobility was a double-valued function of the roughness correlation length, Λ. Here, we show that the indeterminacy of Λ can be overcome by comparing the temperature dependence of the calculated and measured mobilities; hence, we extend the calculation to higher temperatures. While the scattering mechanism itself is temperature independent, the band structure and the carrier distribution are temperature-dependent. As a function of temperature, we find that as a function of the correlation length, mobilities can increase, decrease, or remain constant. This behavior is explained on the basis of the physics of the problem.
    Infrared Physics & Technology 07/2013; 59:60–63. · 1.36 Impact Factor
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    B Ullrich, J S Wang, G J Brown
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    ABSTRACT: We address erroneous statements made by Nordin et al (2012 Nanotechnology 23 275701) claiming the inadequacy of an expression we, Ullrich et al (2011 Appl. Phys. Lett. 99 081901), used to fit the temperature dependence of the photoluminescence and the absorption of PbS quantum dots. We further correct a quote by Nordin et al, who, when referring to our work, mistakenly claimed temperature invariance of the Stokes shift.
    Nanotechnology 06/2013; 24(28):288001. · 3.84 Impact Factor
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    ABSTRACT: We report ternary growth studies to develop a largely strained InAs/InGaSb superlattice (SL) material for very long wavelength infrared (VLWIR) detection. We select a SL structure of 47.0 Å InAs/21.5 Å In0.25Ga0.75Sb that theoretically designed for the greatest possible detectivity, and tune growth conditions for the best possible material quality. Since material quality of grown SLs is largely influenced by extrinsic defects such as nonradiative recombination centers and residual background dopings in the grown layers, we investigate the effect of growth temperature (Tg) on the spectral responses and charge carrier transports using photoconductivity and temperature-dependent Hall effect measurements. Results indicate that molecular beam epitaxy (MBE) growth process we developed produces a consistent gap near 50 meV within a range of few meV, but SL spectral sensing determined by photoresponse (PR) intensity is very sensitive to the minor changes in Tg. For the SLs grown from 390 to 470 °C, a PR signal gradually increases as Tg increases from 400 to 440 °C by reaching a maximum at 440 °C. Outside this growth window, the SL quality deteriorates very rapidly. All SLs grown for this study were n-type, but the mobility varied in a variety of range between 11,300 and 21 cm2/Vs. The mobility of the SL grown at 440 °C was approximately 10,000 V/cm2 with a sheet carrier concentration of 5 × 1011 cm-2, but the mobility precipitously dropped to 21 cm2/Vs at higher temperatures. Using the knowledge we learned from this growth set, other growth parameters for the MBE ternary SL growth should be further adjusted in order to achieve high performance of InAs/InGaSb materials suitable for VLWIR detection.
    Proc SPIE 06/2013;
  • F. Szmulowicz, G. J. Brown
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    ABSTRACT: Superlattice transport has acquired new relevance owing to the current interest in InAs/GaSb and other superlattices (SL) for third-generation infrared detector focal plane arrays. Interface-roughness scattering (IRS) is known to limit carrier mobilities at low temperatures. Whereas horizontal (in-plane) transport measurements are standard, perpendicular transport measurements (across SL layers)—the ones relevant to the operation of infrared sensors—are non-routine and seldom performed; vertical SL transport is also less well studied theoretically. Therefore, we extend our previous work on low-temperature SL transport by studying horizontal and vertical IRS-limited transport in InAs/GaSb SLs as a function of temperature, SL parameters, and the degree of roughness. Electron mobilities are calculated by solving the Boltzmann equation with temperature-dependent bands and carrier screening, and the results are discussed by analyzing the behavior of the relaxation rates and spectral mobilities, defined as mobilities as a function of carrier energy. New computational tools are devised to handle the implicit integral equation for the horizontal relaxation rates. We find that the behavior of the relaxation rates and spectral mobilities undergoes a change for energies below and above the conduction band bandwidth, which dictates the ultimate behavior of mobilities as a function of temperature. The calculated mobilities are found to display a rich variety of behaviors as a function of temperature, either increasing, decreasing, or remaining relatively constant, depending on the correlation length of interface roughness, Λ, and the conduction band bandwidth. Since the horizontal mobility is a double-valued function of Λ, the temperature dependence of mobilities can be used to eliminate this indeterminacy in order to assess the degree of interface roughness.
    Journal of Applied Physics 01/2013; 113(1). · 2.21 Impact Factor
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    ABSTRACT: In order to limit cooling requirements, it is important to operate superlattice devices such as infrared detectors at the highest possible temperatures consistent with maintaining satisfactory figures of merit regarding signal and noise. One of the characteristics governing the device performance is vertical carrier mobility, although only horizontal mobilities are routinely measured. Recently, we calculated low-temperature vertical and horizontal mobilities, as limited by interface roughness scattering, for type-II InAs/GaSb superlattices as a function of SL dimensions and the degree of roughness. We found that the horizontal mobility was a double-valued function of the roughness correlation length, Λ. Here, we show that the indeterminacy of Λ can be overcome by comparing the temperature dependence of the calculated and measured mobilities; hence, we extend the calculation to higher temperatures. While the scattering mechanism itself is temperature independent, the band structure and the carrier distribution are temperature-dependent. As a function of temperature, we find that as a function of the correlation length, mobilities can increase, decrease, or remain constant. This behavior is explained on the basis of the physics of the problem.
    Infrared Physics & Technology 01/2013; 56:76–79. · 1.36 Impact Factor
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    ABSTRACT: Precise quantification of the magnetoelectric coupling strength in surface charge induced magnetoelectric effect was investigated in NiFe/SrTiO3 thin film heterostructures with different ultra-thin NiFe thicknesses through voltage induced ferromagnetic resonance. The voltage induced ferromagnetic resonance field shifts in these NiFe/SrTiO3 thin films heterostructures showed a maximum value of 65 Oe at an intermediate NiFe layer thickness of ∼1.2 nm, which was interpreted based on the thin film growth model at the low-thicknesses and on the charge screening effect at large thicknesses. The precise quantification and understanding of the magnetoelectric coupling in magnetic/dielectric thin films heterostructures constitute an important step toward real applications.
    Applied Physics Letters 01/2013; 103(23):232906-232906-5. · 3.52 Impact Factor
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    B. Ullrich, J. S. Wang, G. J. Brown
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    ABSTRACT: The effect of a laterally applied electric field (≤10 kV/cm) on the photoluminescence of colloidal PbS quantum dots (diameter of 2.7 nm) on glass was studied. The field provoked a blueshift of the emission peak, a reduction of the luminescent intensity, and caused an increase in the full width at half maximum of the emission spectrum. Upon comparison with the photoluminescence of p-type GaAs exhibits the uniqueness of quantum dot based electric emission control with respect to bulk materials.
    AIP Advances 11/2012; 2(4). · 1.35 Impact Factor
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    ABSTRACT: We explore the optimum growth space for a 47.0 Å InAs/21.5 Å Ga0.75In0.25Sb superlattices (SLs) designed for the maximum Auger suppression for a very long wavelength infrared gap. Our growth process produces a consistent gap of 50 ± 5 meV. However, SL quality is sensitive to the growth temperature (Tg). For the SLs grown at 390−470 °C, a photoresponse signal gradually increases as Tg increases from 400 to 440 °C. Outside this temperature window, the SL quality deteriorates very rapidly. All SLs were n-type with mobility of ∼10 000 V/cm2 and 300 K recombination lifetime of ∼70 ns for an optimized SL.
    Applied Physics Letters 10/2012; 101(17). · 3.52 Impact Factor
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    ABSTRACT: Annealing effect on the quality of long wavelength infrared (LWIR) InAs/GaSb superlattices (SLs) has been investigated using atomic force microscopy (AFM), photoconductivity, temperature dependent Hall, and time-resolved differential transmission measurements using an electronically delayed pump-probe technique. Quarters of a single SL wafer were annealed at 440, 480, and 515 °C, respectively for 30 minutes under a Sb-over pressure. Morphological qualities of the SL surface observed by AFM did not show any indication of improvement with annealing. However, the spectral intensity measured by photoconductivity showed an approximately 25 % improvement, while the band gap energy remained at ~107 meV for each anneal, The electron mobility was nearly unaffected by the 440 and 480 °C anneals, however showed the improvement with the 515 °C anneal, where the mobility increased from ~4500 to 6300 cm 2 /Vs. The minority carrier lifetime measured at 77 K also showed the improvement with annealing, increasing from 12.0 to 15.4 nanoseconds. In addition to the longer lifetimes, the annealed samples had a larger radiative decay component than that of unannealed sample. Both the longer measured lifetime and the larger radiative decay component are consistent with the modest improvement in the quality of the annealed SL sample. Overall the qualities of LWIR SL materials can be benefit from a post growth annealing technique we applied.
    SPIE. Optics and Photonics, San Diego, California, USA; 10/2012
  • Microscopy and Microanalysis 07/2012; 18(S2):1808-1809. · 2.50 Impact Factor
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    ABSTRACT: New infrared (IR) detector materials with high sensitivity, multi-spectral capability, improved uniformity and lower manufacturing costs are required for numerous space-based infrared imaging applications. To meet these stringent requirements, new materials must be designed and grown using semiconductor heterostructures, such as quantum wells and superlattices, to tailor new optical and electrical properties unavailable in the current generation of materials. One of the most promising materials is a strained layer supperlattice (SLS) composed of thin InAs and GaInSb layers. While this material shows theoretical and early experimental promise, there are still several materials growth and processing issues to be addressed before this material can be transitioned to the next generation of infrared detector arrays. Our research is focused on addressing the basic materials design, growth, optical properties, and electronic transport issue of these superlattices.
    International Journal of High Speed Electronics and Systems 04/2012; 10(01).
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    ABSTRACT: The impact of post growth annealing on the electrical properties of a long wavelength infrared type-II superlattice (SL) was explored. Quarters of a single SL wafer were annealed at 440 °C, 480 °C, and 515 °C, respectively for 30 min. Changes in the electrical properties were followed using spectral photoconductivity, temperature dependent Hall effect, and time-resolved pump-probe measurements. The bandgap energy remained at ∼107 meV for each anneal, and the photoresponse spectra showed a 25% improvement. The carrier lifetime increased from 12 to ∼15 ns with annealing. The electron mobility was nearly constant for the 440 °C and 480 °C anneals, and increased from ∼4500 to 6300 cm2/Vs for the 515 °C anneal.
    Journal of Applied Physics 03/2012; 111(5). · 2.21 Impact Factor
  • B Ullrich, G J Brown, H Xi
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    ABSTRACT: To determine the band gap of bulk semiconductors with transmission spectroscopy alone is considered as an extremely difficult task because in the higher energy range, approaching and exceeding the band gap energy, the material is opaque yielding no useful data to be recorded. In this paper, by investigating the transmission of industrial GaSb wafers with a thickness of 500 µm, we demonstrate how these obstacles of transmission spectroscopy can be overcome. The key is the transmission spectrums’ derivative, which coincides with the Gaussian function. This understanding can be used to transfer Beers’ law in an integral form opening the pathway of band gap determinations based on mathematical parameters only. The work also emphasizes the correlation between the thermal band gap variation and Debye temperature.
    Semiconductor Science and Technology 01/2012; 27(10). · 1.92 Impact Factor
  • B Ullrich, G J Brown
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    ABSTRACT: Although known for more than twenty years, there is limited information in the literature regarding the experimental parameters of lock-in based phase sensitive emission detection in conjunction with Fourier spectrometers. For this technique - also known as double modulation spectroscopy - we present a guideline for parameter optimization by measuring the photoluminescence of InSb:Te at cryogenic temperatures.
    The Review of scientific instruments 01/2012; 83(1):016105. · 1.52 Impact Factor
  • Journal of Applied Physics 01/2012; 111:053113. · 2.21 Impact Factor
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    ABSTRACT: InAs/GaSb superlattice (SL) materials are of great interest for infrared (IR) detection applications. There is tremendous design flexibility in these superlattices but every design change has an impact on the epitaxial growth conditions for optimized performance. In here, we discuss how a simple design change of InAs width affects the material properties. As the InAs layer thickness increases from 9 monolayers (MLs) to 16 MLs for a fixed GaSb layer thickness of 7 MLs, the spectral intensity measured by photoconductivity decreases by two orders of magnitude, while the calculated absorption strength decreases by less than a factor of two. Since the measured transport properties of mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) SLs were very different-majority carriers in MWIR (LWIR) SLs were holes (electrons)-the large decrease in the photoresponse is due to changes in extrinsic material factors that affect these charge carrier properties.
    12/2011;

Publication Stats

556 Citations
206.37 Total Impact Points

Institutions

  • 2001–2014
    • Air Force Research Laboratory
      Washington, Washington, D.C., United States
  • 1983–2013
    • Wright-Patterson Air Force Base
      Dayton, Ohio, United States
  • 2011
    • Rice University
      • Department of Electrical and Computer Engineering
      Houston, TX, United States
  • 2008
    • University of Dayton
      • Department of Physics
      Dayton, OH, United States
  • 2002–2007
    • National Research Council Canada
      • Institute for Microstructural Sciences (IMS)
      Ottawa, Ontario, Canada
  • 1995–1998
    • Northwestern University
      • Department of Electrical Engineering and Computer Science
      Evanston, IL, United States